The present invention relates to a 6-degree-of-freedom control apparatus for controlling three position axes and three attitude axes, i.e., a total of 6 degrees of freedom of a spacecraft such as an artificial satellite.
Conventionally, to dock a spacecraft such as an artificial satellite and another spacecraft, put them into orbit, and maintain a predetermined orbital position, a 6-degree-of-freedom control apparatus controls three position axes and three attitude axes of the spacecraft. FIG. 4 shows a conventional 6-degree-of-freedom control apparatus disclosed in Japanese Patent Laid-Open No. 7-33095 (reference 1).
Referring to FIG. 4, the 6-degree-of-freedom control apparatus comprises a spacecraft main body 101, a position detector 102 for measuring the position of the spacecraft main body 101, a target position value generation section 103 for outputting the target position value of the spacecraft main body 101, a position control calculation section 104 for calculating a control signal associated with position control of the spacecraft main body 101, an attitude detector 105 for measuring the attitude of the spacecraft main body 101, a target attitude value generation section 106 for outputting the target attitude value of the spacecraft main body 101, an attitude control calculation section 107 for calculating a control signal associated with attitude control of the spacecraft main body 101, a noninterference calculation section 108 for eliminating interference on the dynamics on the basis of the calculation results from the position control calculation section 104 and attitude control calculation section 107, and a feedforward calculation section 109 for compensating the acceleration components of the target position and attitude values and the inertial force on the dynamics.
The spacecraft main body 101 comprises a thruster selection section 116 for selecting a combination of thrusters and thruster jet pattern on the basis of an input control signal, a thruster modulator 117 including a thruster driving circuit, a plurality of thrusters 118, and a spacecraft dynamics 119 that changes depending on the thrusts generated by the thrusters 118.
In the 6-degree-of-freedom control apparatus for a spacecraft shown in FIG. 4, the position and attitude of the spacecraft main body 101 are detected by the position detector 102 and attitude detector 105, respectively. The position control calculation section 104 calculates a control signal associated with position control of the spacecraft main body 101 on the basis of the deviation between the output from the position detector 102 and the target position value output from the target position value generation section 103. The attitude control calculation section 107 calculates a control signal associated with attitude control of the spacecraft main body 101 on the basis of the deviation between the output from the attitude detector 105 and the target attitude value output from the target attitude value generation section 106.
The feedforward calculation section 109 calculates a compensation amount for the acceleration components of the target values on the basis of the outputs from the target position value generation section 103 and target attitude value generation section 106. The feedforward calculation section 109 also calculates the compensation amount for the inertial force on the basis of the outputs from the position detector 102 and attitude detector 105. The noninterference calculation section 108 eliminates interference on the dynamics between the control signal output from the position control calculation section 104 and that output from the attitude control calculation section 107. The output from the noninterference calculation section 108 is added to the output from the feedforward calculation section 109 and then output to the thruster selection section 116 mounted in the spacecraft main body 101.
On the basis of the input control signal, the thruster selection section 116 selects a combination of the thrusters 118 and jet pattern simultaneously for a plurality of axes such that the fuel consumption becomes minimum. The thruster modulator 117 actuates the valves of the selected thrusters 118 of the plurality of thrusters 118 to supply fuel in accordance with the thruster control signal output from the thruster selection section 116. With this operation, the thrusters 118 selectively jet, and the position and attitude of the spacecraft main body 101 are freely controlled.
The thruster selection section 116 selects the combination of the thrusters 118 and jet pattern on the basis of a lookup table and realizes an efficient thruster control method capable of minimizing the total fuel jet amount using the offset jet logic or permutation jet logic. The offset jet logic removes an offset jet pattern that nullifies the resultant force and torque by jet of the selected thrusters 118. The permutation jet logic replaces a thruster jet combination with a combination that minimizes the total jet amount, though the resultant force and torque are generated by the selected thrusters 118.
However, in the 6-degree-of-freedom control apparatus shown in FIG. 4, since the thruster selection section 116 distributes the jet to the plurality of thrusters 118 used for axial control in accordance with the control signal generated on the basis of the position and attitude deviations of the spacecraft main body 101, the thrusters 118 need always be switched. However, the individual thrusters 118 mounted on the spacecraft have a large variation in their output characteristics. Additionally, the variation is random.
Hence, in switching the thrusters 118 used for axial control, it is difficult to accurately grasp the influence of the variation in output characteristics between the individual thrusters 118 on the accuracy of axial control. For this reason, it is hard to accurately control the position and attitude of the spacecraft main body 101.
In the 6-degree-of-freedom control apparatus shown in FIG. 4, after all control signals associated with the axes are added, the thruster selection section 116 selects thrusters to be used, and the thruster modulator 117 executes jet modulation in units of thrusters. The thruster selection section 116 optimizes the thrusters to be used in accordance with, e.g., the required thruster jet amount, independently of the state of the spacecraft main body 101. For this reason, the relationship between thruster jet and the axial motion of the spacecraft main body 101 is unclear, and the force generated by thrust jet can hardly be decomposed in units of axes.
Modulation executed by the thruster modulator 117 substantially corresponds to the axial motion of the spacecraft main body 101. In the arrangement shown in FIG. 4 wherein thruster jet and the motion of the spacecraft main body 101 cannot be associated with each other, the modulation logic to be executed by the thruster modulator 117 cannot be set in advance.
In the 6-degree-of-freedom control apparatus shown in FIG. 4, information associated with the velocity/angular velocity of the spacecraft main body 101 is not used for axial control. Hence, for position and attitude control of the spacecraft main body 101, phase lead compensation cannot be achieved, resulting in limited control performance.
In a thruster control method disclosed in Japanese Patent Laid-Open No. 62-59200 (reference 2), for simultaneous control of a plurality of axes, 6-degree-of-freedom control of a spacecraft is realized by simply adding control logic components for individual axes.
However, since the logic components for axial control are only simply added, fuel consumption of thrusters cannot be suppressed.
It is an object of the present invention to provide a 6-degree-of-freedom control apparatus for a spacecraft, which can simultaneously realize accurate position and attitude control of the spacecraft and suppression of fuel consumption.
In order to achieve the above object, according to the present invention, there is provided a 6-degree-of-freedom control apparatus for a spacecraft, comprising a plurality of thrusters for controlling three position axes and three attitude axes of a spacecraft by jet, thruster driving means for selectively driving the thrusters on the basis of a thruster control signal, position/velocity detection means for measuring a position and velocity of the spacecraft, target position/velocity generation means for generating target values of the position and velocity of the spacecraft, attitude/angular velocity detection means for measuring an attitude and angular velocity of the spacecraft, target attitude/angular velocity generation means for generating target values of the attitude and angular velocity of the spacecraft, and 6-degree-of-freedom control means for generating the thruster control signal on the basis of a deviation between an output from the position/velocity detection means and an output from the target position/velocity generation means and a deviation between an output from the attitude/angular velocity detection means and an output from the target attitude/angular velocity generation means and outputting the thruster control signal to the thruster driving means.